Flemming Buus Bendixen

Controlling the DC-link midpoint potential in a six-phase motor-drive

Traditionally electrical motors have three phases, but multiphase motors have shown to improve motor performance and efficiency. This paper concentrates about the control algorithm for a six-phase induction motor with third harmonic current injection. The problem is that typically a seventh inverter branch and filter inductance is needed for stabilizing the midpoint potential of the series connected DC-capacitor link. A new control strategy that pre-calculates the allowed voltage ripple and controls the motor voltage accordingly (using two standard three phase inverter modules) is suggested. With this new control strategy the seventh branch and an inductance can be saved. It also opens the possibility to use two standard three-phase inverters to supply the six-phase motor. An experimental setup is build and the theory is verified in the test case. The proposed control strategy works satisfactory. A drawback is that the voltage ripple in the midpoint increases, if a third harmonic current is injected specially at low speed, which could demand a higher voltage rating of the capacitors. Another drawback is that the higher voltage ripple will stress the capacitors hence the lifetime may be shortened.

Characterization of 50/60 Hz current transformers from 20 Hz to 10 kHz

Characterization of five accuracy class 0.2 current transformers rated 600 A up to 2500 A is done by measurement of phase displacement and amplitude error as function of frequency and current. The test system is presented and the test procedure is described. The accuracy of test system voltage and current measurements is addressed and separate measurements on the transformer burden (load resistance) are carried using other test equipment. This enables an assessment of the test equipment measurement ability. Finally, measurements of the current error and phase displacement of five different current transformers with burden are carried out